1. Field of the Invention
The invention relates to an aqueous ink for the forming of electrodes for an electrochemical cell operating at high temperature. Such an aqueous ink comprises a mineral filler, a binder, and a dispersant.
The field of application especially relates to the manufacturing, by screen printing, of ceramic components for electrochemical cells intended for high-temperature fuel cell applications (SOFC, “solid fuel cells”) and for high-temperature electrolysis of water vapor (EHT or SOEC, “solid oxide electrolyzer cell”).
2. Description of Related Art
Ceramic electrochemical cells dedicated to high-temperature applications (600-1,000° C.) may have a so-called electrolyte supported cell configuration. In this embodiment, the thick electrolyte (120-200 micrometers) also fulfils the function of mechanical support for the entire cell; the electrodes (anode and cathode) being generally arranged on either side of the electrolyte according to the following concept:                yttria-stabilized zirconia (YSZ) electrolyte;        so-called hydrogen electrode (anode in SOFC use or cathode in EHT) comprising a mixture of yttria-stabilized zirconia (YSZ) or of ceria substituted with yttria (CYO) or gadolinium oxide (CGO), and nickel oxide (NiO). The nickel oxide is reduced at the starting of the cell to form metal nickel and thus form a cermet (sintered composite material formed of at least a metal and a ceramic product);        air electrode (cathode in SOFC use or anode in EHT) typically comprising a mixture of yttria-stabilized zirconia (YSZ) and of lanthanum strontium manganite (LSM).        
Generally, the electrodes comprise (a) a so-called functional layer where the electrochemical reaction takes place and (b) a so-called collector layer in charge of collecting (in SOFC mode) or supplying (in EHT mode) the current.
This electrochemical cell manufacturing method has especially been described by Huijsmans et al. (Fuel Cells Bulletin, Volume 2, Issue 14. November 1999, Pages 5-7). The electrolyte is first formed by strip casting before being sintered (densified) at 1,500° C. Its thickness may vary from 80 to 250 micrometers. Then, the two electrodes of lower thickness (≈50 μm) are deposited on either side of the electrolyte by screen printing, and simultaneously sintered at a temperature ranging between 1,200 and 1,300° C.
Other thin film (a few tens of microns) forming methods have been described. These especially are suspension spraying, spin coating, or suspension coating. These methods require an ink or a ceramic suspension containing the electrode materials in the form of powder. According to these embodiments, the formulations of the suspensions or ceramic inks are generally comprised of:                the mineral filler: LSM, LSM/YSZ mixture, NiO, NiO/YSZ mixture (or CYO, CGO), YSZ (or CYO, CGO);        an organic solvent: terpineol, alcohol, methyl ethyl ketone;        an organic binder in charge of the bonding between the ceramic particles forming the electrodes and the electrolyte before sintering: ethyl cellulose, polyvinyl butyral.        
The screen printing quality of the electrode depends on the rheological behavior of the ink. The ink must especially have a shear rate thinning or pseudoplastic behavior, that is, an infinite viscosity in the absence of shearing to avoid its flowing after printing and a low viscosity in the presence of shearing to ease the passing of ink through the printing screen (FIG. 1).
Typically, for a fine-quality printing, the ink viscosity must range between 3 and 20 Pa·s for the shearing imposed by the screen printer (from 40 to 700 s−1). Thus, the electrode has advantages, among which the absence of surface defects (homogeneous thickness), an easy screen-substrate separation (no bubbles), and the fact that the ink does not spread after the printing can be noted.
However, independently from the organic formulation, the drying phase causes the total volatilization of the solvents and thus the emission of irritant vapors. At an industrial production scale, tons of vapor may be emitted, which require the provision of an installation of vapor collection and condensation for a possible subsequent reuse.
Electrode manufacturing methods implying the use of organic inks, as described in document WO2006/066973, thus have non-negligible disadvantages especially due to the use of organic solvents.
The use of aqueous inks comprising the same quantity of mineral filler as prior art organic inks has up to now mainly run up against the incompatibility between the viscosity of said aqueous ink and screen printing deposition.
The Applicant has however developed an aqueous ink having a rheological behavior comparable to that of an organic ink without for all this significantly modifying the filler rate. Further, the mineral filler dispersion is improved in aqueous phase with respect to organic solvents. The aqueous ink according to the present invention further avoids having to use installations for capturing and condensing organic solvent vapors, without altering the cell component performances.